Problem is that you have two different sql configuration databases. User is activated on a BES that connects to db A (it is written in user's mailbox) and now you want to activate it on db B and another BES server.
Solution:
Remove user from old BES and delete bb info from user mailbox when prompted.
Then add him to new bes and activate it on new BES.

wipping is a MUST even when you don't have a problem. I would also remove his user account from new BES and clear all (old and new) bb information from his mailbox when prompted. Otherwise there might be some leftover referances in his mailbox about old server UID. Wait until user acct. dissapear from User pending delete tab and then simply re-add him back. Wipe and activate. Should work.